No, this is the only correct word order: *a(n) so severe accident would be ungrammatical!
The rule here is that where we wish to modify a noun (such as 'accident') with an adjective phrase of the form [so + ADJ], there are two ways in which it can be done:
1. using 'so', we follow the pattern [so+ ADJ +a(n)+ NP]
2. using 'such', we instead say [such+ a(n)+ ADJ+ NP], giving 'such a severe accident' as the alternative formulation.
The second is actually the more commonly heard, the first being rather formal.